This invention relates to a system for monitoring a capstan drive to control tension in a cable delivered to the capstan from a reel carried on a land vehicle which is propelled along a cable placement site, and more particularly, to such a system in which the land vehicle includes an implement to expose a subgrade for the simultaneous placement of the cable passing from the capstan for delivery from the vehicle.
It is a well-known practice to place cable underground, particularly telephone communication cable and electric utility cable along a track of land to supply a particular utility to a consumer. Sometimes a cable is pulled through an underground duct while, in other instances, the cable is placed directly in the ground at a subgrade without the use of a duct to form an enclosure. A land vehicle, such as a tractor, can be equipped with a plow to penetrate the soil to depths of 36 to 60 inches depending on the soil conditions, the protection required and other engineering requirements. Directly behind the plow, a relatively narrow opening is produced in the soil which is sufficient to feed a cable downwardly to the depth of the exposed subgrade. At a point immediately behind the plow there is a structure used to direct the cable beneath the ground level. The soil is allowed to collapse about the plowed opening whereby it is usually unnecessary to perform a backfilling operation along the plowed course. Procedures are, however, necessary to protect the cable from damage during the underground placement operation. Special handling requirements are usually specified by the manufacturer of the transmission cable. These requirements include a need to monitor the tension applied to the cable while it is being plowed into the underground placement site. The nature of such a cable placement operation requires the use of a tractor having a large tractive capacity which is necessary to pull the plow along a track at a depth to the desired subgrade for burying the cable. For this purpose, it is necessary to utilize a tractor having a pull capacity of 50,000 to 75,000 pounds.
The need to exercise special handling during placement of cable is particularly acute when placing communication cable using optical fibers rather than metal wires as a transmission medium. By way of a specific example, manufacturers of optical fiber cable typically specify that the cable can be subjected to no more than 400 to 1000 pounds of tension in a longitudinal tensile pull load and/or when wound about a radius such as a capstan wheel. The relatively high pull load capacity of the tractor used for the plowing operation and the relatively low limit to the maximum tension which can be imposed on most transmission cable, place several restrictions on the ability of an operator to monitor the cable tension while the cable is plowed into a subgrade. Should the actual tension on a specific portion of the cable approach the maximum allowable tension, almost immediate action is necessary to prevent cable damage. One manner for preventing cable damage comprises a stoppage of advancement by the tractor; however, it is not possible for an operator of the tractor to intuitively determine tension on the cable or continuously monitor cable tension because of the massive power capacity of the tractor as compared to the relatively low limit to the maximum tension which can be imposed on the cable.
The development of tension on the cable during the placement operation can be intentionally developed. Tension of the cable can also develop as the result of diverse influences occurring during the cable placement operation. For example, it may be necessary to steer the tractor or other vehicle along a course of travel which is curved, i.e., not linear. The vehicle may travel about a radius to which the plow is remote whereby a greater or lesser amount of cable is drawn from the vehicle, depending on whether the course of travel is concave or convex. Tension on the cable may undergo a dramatic increase should, for example, the supply reel come into contact with tree branches or other obstruction which prevents normal feeding of the cable from the reel. Tension on the cable can also change with a change to speed at which the vehicle advances along the ground. Thus, for example, should the speed of the vehicle increase, the tension on the cable will also increase, assuming that a constant drag was maintained on the cable supply reel carried by the vehicle.
In my U.S. Pat. No. 4,461,459, there is disclosed a tension meter which is connected between a pull line and a cable to provide a signal corresponding to the tension imposed on the cable by the pull line during placement in an underground duct. The tension signal is used to control the tension imposed on the pull line by a motor for a winch located at the end of the duct. However, because of the nature of the cable placement operation using a plow and eliminating a duct, such a tension measuring device cannot be utilized. The present invention utilizes a capstan to tension the cable passing along the vehicle between a supply reel and the entry site into an exposed subgrade. The drive system for the capstan includes a monitoring system responsive to tension in the cable upstream and/or downstream of the capstan.